I'll investigate those Firefox issues too - I did most of the testing in Chrome, Safari and IE, but will do some more on FF to ensure it works OK on all browsers.
If traffic gets really high on our site, I may resort to locally hosted images to reduce load time. Unsplash.it may also be throttling the image load time for us, as I am thinking the sudden spike from this HN post may have alerted them to the fact that there is suddenly a flurry of remote calls to them from our server IP? Or perhaps the Amazon CloudFront CDN is causing a slow down there? I will investigate - thanks for your useful feedback.
In fact, I may be able to go back to using coloured Unsplash images. I went for Grayscale images purely so that I could stick to white text for the contrast.
I already use pre-downloaded Unsplash images within the app itself for the user 'lock screens'. I've curated and placed about 20 images on our server expressly for that purpose [1],[2]
[1] - https://flic.kr/p/NV8cS3 [2] - https://flic.kr/p/NN3vvY
http://i.imgur.com/YNBEUzc.jpg
Also, it gives some strange behavior in Firefox. If I load the below in a tab, it shows a random image as expected. Then if I open it again in a new tab it shows the previous image that loaded from the first tab, and then it switches images to a new random one a second later.
http://www.hrpartner.io/nopage
Lastly, you could solve the above issues, and decrease the load time from 1s to 200ms if you just self hosted a few random images, and you wouldn't have to worry about your 404 page depending on Unsplash.
Edit: It's actually taking 6-8 seconds to load the image now, and the 404 page is white text on a white background during that time.